Concurrent access was my first idea too.
To test it: I erased the flash to standard boot and main. Then I preloaded the SD with data. Re-ran the FTP test and it still had the same crashes.
Using the FTP server on the device is for me is the easiest way.
Otherwise I would need to create a Timer to upload data at regular intervals. To keep data transfer low I would need keep the intervals long avoiding me to have the flexibility of choosing which data I want to download and when.
To be honest, I'm a bit disappointed by the eagerness of pycom to attack this problem.
The same with my other crash report.